The present invention relates to a nematic liquid crystal composition being particularly suitable for an active matrix type liquid crystal display element and having a negative dielectric anisotropy value, and a liquid crystal display element comprising this liquid crystal composition.
In liquid crystal display devices (LCD), such advantages as low consumption of electric power, miniaturization and weight reduction can be obtained in contrast to a CRT (Braun tube-type display), and the LCD has practically been used in various modes such as twisted nematic (TN) mode, supertwisted nematic (STN) mode and thin film transistor (TFT) mode. Above all, active matrix LCD (AM-LCD) such as TFT has been noticed as a prospective winner of flat display along with the progress of colorization and high precision.
For this AM-LCD liquid crystal composition, following characteristics are required:
1) A high voltage-holding ratio (VHR) which can maintain high contrast of the LCD,
2) A wide range of nematic liquid crystal phase which can comply with the change of its use environment,
3) Being able to take suitable optical anisotropy (xcex94n) in accordance with a cell thickness, and
4) Being able to take suitable threshold voltage in accordance with a driving circuit.
As an operating mode of the AM-LCD, there has been mainly used TN display mode wherein alignment of liquid crystal molecules between upper and lower electrode substrates is twisted by 90xc2x0, but since the view angle of the mode is narrow, the mode has a drawback that the application thereof to a large screen display has been difficult.
Accordingly, the following techniques have been proposed for the modes to improve their view angle:
a) IPS display mode wherein liquid crystal display devices exhibit a homogeneous alignment state when voltage is not applied, and a liquid crystal molecule rotates by 45 to 90xc2x0 in a plane when the voltage its applied (R. Kiefer, B. Weber, F. Windscheid and G. Baur, xe2x80x9cIn-Plane Switching of Nematic Liquid Crystalsxe2x80x9d, JAPAN DISPLAY ""92, p. 547), and
b) VA display mode wherein a liquid crystal display devices exhibit a homeotropic alignment state when voltage is not applied, and changes into an alignment state in one horizontal direction when the voltage is applied (K. Ohmuro, S. Kataoka, T. Sasaki and Y. Koike, xe2x80x9cDevelopment of Super-High-Image-Quality Vertical-Alignment-Mode LCDxe2x80x9d, SID 97 DIGEST, p. 845).
These display modes are characterized by a high-speed response and a high contrast in addition to the realization of the broad view angle, and they also have a large feature that liquid crystal compositions having a negative dielectric anisotropy value (xcex94xcex5) can be applied. These operating modes utilize an electrically controlled birefringence mode.
In this case, in order to obtain the optimum contrast, it is necessary to set a product xcex94nxc2x7d of the optical anisotropy value (xcex94n) by the cell thickness (d) to a certain value, for example, about 0.275 xcexcm, but since the cell thickness (d) usually ranges from 3 to 6 xcexcm, the xcex94n value is required to be in the range of about 0.05 to 0.10.
Response time is proportional to a value of viscosity (hereinafter, abbreviated to xcex7 in some cases) of the liquid crystal composition, and hence, in order to achieve high-speed response, the liquid crystal composition having a small value of xcex7 is required. Furthermore, it is preferred that the threshold voltage decreases as the absolute value of the dielectric anisotropy (hereinafter, abbreviated to xcex94xcex5 in some cases) increases. Therefore, the liquid crystal composition having a negative and absolutely large value of xcex94xcex5 is required.
In order to meet such requirements and other requirements in compliance with various objects, liquid crystal compositions have intensively been investigated so far but they are believed to be still out of the expectation.
An object of the present invention is to provide a liquid crystal composition having a suitable value of xcex94n applicable to the above-mentioned display mode a) and b), a low viscosity, a negative and absolutely large value of xcex94xcex5, a broad view angle and a wide nematic liquid crystal phase range, and thus having a high voltage-holding ratio and other various properties required for the above AM-LCD liquid crystal composition.
For achieving the object described above, the present invention is summarized as follows:
(1) A liquid crystal composition comprising, as a first component, at least one compound selected from the compounds expressed by the general formula (I), and, as a second component, at least one compound selected from the compounds expressed by the general formulas (II-1) and (II-2): 
wherein R1, R3 and R5 each independently represent an alkyl group having 1 to 10 carbon atoms or an alkenyl group having 2 to 10 carbon atoms; R2, R4and R6 each independently represent an alkyl group or an alkoxy group having 1 to 10 carbon atoms, or an alkenyl group having 2 to 10 carbon atoms; Z1 to Z5 each independently represent a single bond or xe2x80x94CH2CH2xe2x80x94; rings A1 and A2 each independently represent a 1,4-phenylene group or a trans-1,4-cyclohexylene group; and in the case that the ring A2 is the 1,4-phenylene group, at least one hydrogen atom at a lateral position of the ring may be substituted by a fluorine atom.
(2) A liquid crystal composition as recited in paragraph 1 wherein the amounts of the first component and the second component are 3 to 40% by weight and 3 to 70% by weight based on the total weight of the liquid crystal composition, respectively.
(3) A liquid crystal composition as recited in paragraph 1 or 2 wherein the liquid crystal composition further comprises, as a third component, at least one compound selected from the compounds expressed by the general formulas (III-1), (III-2), (III-3), (III-4), (III-5) and (III-6): 
wherein R7, R9, R11, R13 , R15 and R17 each independently represent an alkyl group having 1 to 10 carbon atoms or an alkenyl group having 2 to 10 carbon atoms; R8, R10, R12, R14 and R16 each independently represent an alkyl group or an alkoxy group having 1 to 10 carbon atoms, or an alkenyl group having 2 to 10 carbon atoms; R18 represents an alkyl group having 1 to 10 carbon atoms in which one xe2x80x94CH2xe2x80x94may be replaced by xe2x80x94Oxe2x80x94, or an alkenyl group having 2 to 10 carbon atoms; and Q represents a hydrogen atom or a fluorine atom.
(4) A liquid crystal composition as recited in the paragraph 3 wherein the amounts of the first component, the second component and the third component are 3 to 40% by weight, 3 to 70% by weight and 1 to 80% by weight based on the total weight of the liquid crystal composition, respectively.
(5) A liquid crystal composition as recited in the paragraph 3 or 4 wherein the liquid crystal composition further comprises, as a fourth component, at least one compound selected from the compounds expressed by the general formulas (IV-1) and (IV-2): 
wherein R19 and R21 each independently represent an alkyl group having 1 to 10 carbon atoms or an alkenyl group having 2 to 10 carbon atoms; R20 and R22 each independently represent an alkyl group or an alkoxy group having 1 to 10 carbon atoms, or an alkenyl group having 2 to 10 carbon atoms; Z6 to Z8 each independently represent a single bond or xe2x80x94CH2CH2xe2x80x94; and a ring B represents a 1,4-phenylene group or a trans-1,4-cyclohexylene group.
(6) A liquid crystal display element comprising a liquid crystal composition as recited in any one of the paragraphs 1 to 5.